The biggest risk with mixing is the impact on the Winter rating. Different PPD dosage rates necessary for different base oil blends (and to some extent the same for VII treat) means that it's possible that a 5w-30 mixed with a 5w-20 puts you at a 10w-30 or even a 15w-30 or 10w-20 or 15w-20...etc depending on the ratio of the oils mixed and interaction of those components. A majority PAO lube with next to no PPD's mixed with a Group II+ one that's chalk-full, the resultant base oil blend may require more PPD's than is present in the mix is a hypothetical example of one such scenario (not sure how that aligns with reality, one of the resident tribologists could better advise on the specifics of such a scenario).
